1. Field of the Invention
The present invention relates to a two-component developer for use in the electrophotography. More particularly, the present invention relates to a two-component developer comprising a magnetic carrier having a carbon black-containing resin-coating layer and toner composed of a carbon black-containing resin composition.
2. Description of the Related Art
A two-component developer comprising a magnetic carrier and a toner is widely used as the developer in a commercial electrophotographic process. A product obtained by granulating a composition comprising a coloring agent and other toner additives dispersed in a binder resin to a predetermined particle size by pulverization-classification, spray granulation, suspension polymerization, dispersion polymerization or the like is generally used as the toner. For a toner to be used for an office copying machine, in order to control the coloration and the electric resistance, carbon black is generally incorporated in an amount of 5 to 15% by weight into the resin.
Iron powder, ferrite and the like are used as the magnetic carrier, and in order to control consumption of the carrier, the antistatic property and the electric resistance, the magnetic carrier is coated with a resin, and it is known that carbon black is incorporated into this coating of the carrier. For example, Japanese Unexamined Patent Publication No. 61-260254 discloses a carrier for developing an electrostatic latent image, comprising a carrier core having the surface coated with a silicone resin containing carbon black comprising a mixture of furnace black and acetylene black. Furthermore, Japanese Unexamined Patent Publication No. 62-182759 discloses a carrier for developing an electrostatic latent image, which comprises a carrier core having the surface coated with a silicone resin containing carbon black surface-treated with an aminosilane coupling agent or the like.
Resins used for coating carriers have merits and demerits. For example, the above-mentioned silicone resin is excellent in the durability but is defective in that the electric resistance is high, rise of charging is slow and the chargeability is relatively unstable. A fluorine resin is excellent in the consumption resistance, but in view of the frictional charging system, the fluorine resin can hardly be used for a magnetic carrier for a negatively chargeable toner. A styrene resin and an acrylic resin are most excellent in the charging properties, but they are poor in the durability and insufficient in the consumption resistance.
It is deemed that the significance of the above-mentioned proposals resides in that the electric resistance of the coating resin layer of the carrier is adjusted by incorporating carbon black in the coating resin layer. However, in a two-component developer, charging of the toner is greatly influenced by frictional charging properties and electric characteristics of not only the coating layer of the magnetic carrier but also the resin composition constituting the toner. Accordingly, with respect to both of the magnetic carrier-coating layer and the toner, an optimum combination of the frictional charging properties and the electric characteristics should be selected.